1. Field of the Invention
The present invention relates to stress testing of materials, and particularly to a system for testing stress corrosion cracking (SCC) in metallic objects for monitoring purposes.
2. Description of the Related Art
Major industries, such as the oil and gas industry, the petrochemical industry, as well as desalination plants and power plants, experience stress corrosion cracking (SCC) problems. SCC refers to the growth of a crack formation in a corrosive environment and can result from a combination of factors, such as a susceptible material, a corrosive environment, and tensile stresses above a threshold level for that particular material. Various materials, such as metals, polymers, and ceramics, are susceptible to SCC, and the most common premature failures in these materials are typically the result of SCC. SCC of in-service components can occur in areas that are undetectable and/or difficult to access. Further, SCC is considered to be one of the most dangerous forms of failure due to the presence of stress and a corrosive environment, since the crack resulting from corrosion may propagate undetected and cause a leak or a sudden failure, resulting in catastrophic results. To counter such potential dangers, a complete shutdown may be necessary for maintenance and repair. Such actions can incur prohibitive costs, both from loss of production and maintenance that may be required.
Since cracking and failures in the vessels and in pipework in processing plants are ongoing problems, and since plant integrity is a major concern with regard to safety and the environment, engineers and designers must carefully assess various different materials to be used in making the components for an industrial environment. For example, engineers must determine the stress intensity factor (KI) and the threshold stress intensity (KISCC) in the components made from those materials in order to determine the life expectancy of the components in corrosive environments, conformance to construction standards, and the ability of the component to meet performance demands.
Currently, there are various types of systems and methods used to investigate failed components and monitor SCC. For example, typically a testing unit involves the use of a specimen of material placed in a rig and exposed to tensile stress to measure the KI and the KISCC. However, the traditional fracture mechanic techniques require testing rigs that tend to be bulky and require a relatively large amount of space. The use of traditional testing rigs can be prohibitively expensive and time-consuming, both in terms of the device itself and the necessary upkeep and specialty needs. Further, conventional fracture specimens, such as compact tension (CT) specimens and pre-cracked double cantilever beam (DCB) specimens, are relatively expensive and bulky, requiring a rather large thickness to achieve plane strain conditions. Moreover, in some instances, these types of specimens cannot be obtained from failed components.
Therefore, the expense of both the machine and the specimens, as well as the large size of the traditional testing rigs can restrict their use for SCC monitoring. Further, many of the fracture mechanic techniques have been developed to examine SCC in the presence of stagnant (no flowing) water, which is not an actual condition in an industry where the fluid is usually in a flowing condition.
Thus, a system for testing stress corrosion cracking solving the aforementioned problems is desired.